Non-consumable anode for electrolysis

ABSTRACT

The invention relates to a non-consumable anode for electrolysis that contains carbon. Said anode is made of pyrocarbon (pyrographite). The pyrocarbon (pyrographite) anode is stable even in nitrate electrolytes and does not contaminate the electrolyte with erosion products.

FIELD OF THE INVENTION

This invention relates to electrochemistry, particularly to electrolytic-cell anodes containing carbon and stable in working electrolytes.

BACKGROUND OF THE INVENTION

Non-consumable anodes are widely used in electrochemical processes, particularly in electrosynthesis, extraction processes and in the manufacturing of powder products. Sulphate solutions are mostly used in hydroelectrometallurgy due to the satisfactory stability level of platinized titanium and silver, or antimony alloyed lead anodes. Less applicable are chloride solutions despite their higher conductivity that can intensify the process. The reason is the low stability of graphite anodes and contamination of the electrolytes with their erosion products. The use of nitrate solutions is limited due to the lack of anodes that are stable in this medium, and is restricted only to the refining process [1].

In order to increase the graphite anodes' stability, they are impregnated with various materials. In this case we observe the solution contamination with impregnation products which precipitate on the filtration diaphragm and block its pores, thus reducing its service life [2]. There was a successful effort to use pyrocarbon as an alloying additive to the graphite anode for a better service life. The introduction of 5-10% pyrocarbon into the graphite composition made the anode consumption in chloride solutions almost two times lower [3]. But it was not implemented into practice: the gain was not worth the extra costs, and this useful idea was not promoted any further. There was also no significant change in the properties, and the solution's nitrate zone was still resistant to this material. Glassy carbon electrodes are used in chloride and cryolite-alumina melts, as well as in a mixed melt of lithium and potassium chlorides with uranium oxychloride additives [4]. Their use is limited due to their low conductivity compared to regular graphite. In some chloride-based processes graphite is replaced by the so called ORTA anodes (oxide-ruthenium-titanium anodes) [5].

The metal oxide anodes were invented with a pyrocarbon conductive base (!) [6]. However, pyrocarbon was used only as a conductive material, and the active layer of basic metal oxides acted as an anode. Metal oxide anodes are known to be very demanding in terms of their working environment—they cannot resist the change of polarity, short circuits and even temporary process shutdowns [2]. The patent [7] describes the abrupt increase of stability of the prebaked carbon anode intended for the fluoride electrolytic bath. The author relates the stability growth to the generation of the pyrocarbon phase between coke grains during baking at temperatures above 1300° C., and a baking time of over 140 hours. This material containing pyrocarbon was given the name anhalite. The patent [8] is providing the anode for galvanic processes, made of pyrocarbon-impregnated carbon fabrics. However, the low conductivity of the carbon fabric-based anode affected the regularity and thickness of the electroplated coating, and so the authors proposed the anode design with variable thickness of fabric carcass in order to balance the anode-solution current density over the length of the anode.

The patent [9] makes reference to the use of pyrographite, along with graphite and platinum, as an anode in the laboratory processes of the synthesis of hydrocarbons. However, the target goal, i.e. the enhancement of the synthesis output and selectivity, was achieved by choosing the original reagents and using the anodes made of precious metals. Pyrographite was able to provide the hydrocarbons conversion only at the level of 12%, whereas Pt—10%-Ir−42% in the form of alloy, and over 47% in the form of nanoparticles on glassy carbon. Besides, the use of pyrographite was accompanied by the formation of a polymer film on the electrode, which blocked the reaction. No data is provided regarding the comparative stability of anodes, as such goal was not set in the conditions of the laboratory experiment. The patent [10] also makes reference to the use of pyrocarbon as an anode along with other carbon-containing materials, platinum, and other metals. The target goal, i.e. to develop a cost-effective method of erythrol (sugar substitute) extraction, is unlikely to be achieved by using expensive pyrocarbon or platinum, especially as graphite foil is specified as the preferred material for the anode. The stability of anodes made of the above materials was also not taken into consideration due to the small-scale level of investigations.

Based on the available materials from prior art, the conclusion can be made that there is no technically or economically feasible proposal for the use of pyrocarbon (pyrographite) anode in the electrochemical processes. Therefore the share of graphite as anode material in big electrochemistry is still rather significant, as are the efforts to improve its consumer properties. For example, it was proposed to alloy the graphite with silicon using the powder mixing technology—pressing—sintering [11] (prototype). That anode was used for the electroactivation of drinking water, and was able to improve the quality of drinking water of various compositions by electrolytic treatment, and its enrichment with silicon ions. Actually, the prototype material is an example of the silicified graphite produced by our industry. However, pyrographite after several test trials just to improve the anode stability of traditional electrode materials has never been proposed to be used for its major role, i.e. as the anode in electrochemical production.

SUMMARY OF THE INVENTION

It is a technical goal of the present invention to improve the performance of the carbon-containing anode for electrolysis, due to the lower level of anode corrosion and electrolyte contamination.

During the development of the corrosion stable anode, a number of carbon-containing materials were tested as anodes, including fiber, gasproof materials, i.e. graphite grade FDG (fine-grained dense graphite), glassy carbon and pyrocarbon (pyrographite [12]), as well as industrial samples of silicified graphite. The most corrosive environment for anodes was used, i.e. a nitrate electrolyte composed of a water solution of silver nitrate (10%) and nitric acid (1%). The common disadvantage of all tested materials, except the pyrocarbon (pyrographite), was that during the electrolysis process the anodes made of those materials were subject to molecular disintegration. The desintegration products could not be trapped by a filtering diaphragm and contaminated the electrolyte, thereby affecting the purity and quality of the electrolysis product.

The pyrocarbon (pyrographite) nonconsumable anode achieved the target goal.

EMBODIMENTS OF THE INVENTION

Different samples of pyrocarbon (pyrographite) received in NIIGraphite were used for the production of the test anodes. The pyrocarbon anodes were tested in nitrate electrolyte in combination with the consumable anode made of silver purity level 99.99% in conditions of electrolytic process for the production of silver powder grade PSr1, intended for the production of electric contacts. Both anodes were connected to the positive pole of an electrolyser power source. The current supplied to the nonconsumable anode was 10÷15 times lower than that supplied to the consumable anode. The current density on the nonconsumable anode was maintained on the level of 10 A/dm² within a period of several days. The electrolyte remained clear and clean. The production testing of the contacts made from the test powder PSr1 received with the use of the pyrocarbon (pyrographite) non-consumable anode, confirmed the high quality of the powder.

THE PREFERRED EMBODIMENT OF THE INVENTION

None of pyrocarbon samples contaminated the solution. However, it was noted that the pyrocarbon samples with a higher density demonstrated better stability compared to the samples with a lower density. The pyrocarbon grade UPA-3 (pyrolytic reinforced carbon) produced by the Novocherkassk electrode factory was chosen for the manufacturing of the industrial non-consumable anodes [13]. These anodes were put into operation in 2003 and are still successfully used in the electrolysis area of the silver powder production shop at one of the leading RF companies producing electric contacts. The anode erosion (0.12÷0.15% from deposited silver) is mainly of an electromechanical nature due to the fact that ‘interflake’ material bonds burn up under the effect of current and anions (OH⁻ and NO₃ ⁻)*, resulting in flakes' falling and being trapped by the anode filtration diaphragm without contaminating the solution. *-author version.

INDUSTRIAL APPLICABILITY OF THE ANODE

Many years of positive experience of the pyrocarbon (pyrographite) nonconsumable anodes' industrial use in the most corrosive nitrate electrolyte proving their extraordinary stability allow us to recommend such anodes for the use in those electrochemical production facilities where it is economically feasible. The raw material is not cheap but can compete with platinum metals in terms of price and quality, and with multilayer metal-oxide compounds in terms of usage conditions. Moreover, the use of such anodes will expand the possibilities for electrolysis development in nitrate media. And the pyrocarbon production growth will result in its lower cost, thus promoting its use in big electrochemistry.

CITATIONS

1. Applied electrochemistry. College textbook. Edited by A. P. Tomilova, Moscow, ««Chemistry»», 1984.

2. L. M. Yakimenko. Electrode materials in applied electrochemistry. Moscow, ««Chemistry»», 1977.

3. Material for the production of anode intended for the use in chlorine electrolysis. E. M. Ostroumov, L. K. Kosterina, and others. AS USSR 511387, published on 26 Jun. 1977.

4. Glassy carbon. Production. Properties, application. V. D. Chekanova and A. S. Fialkov. Success of chemistry, AS USSR, issue 5, 1971. Tome XL, pg. 803.

5. ORTA anodes. www.rutteh.ru., 2018.

6. Low-consumable anode. H. I. Kavardakov, U. D. Khramtsov and V. I. Kichigin, SU 1668480, published on 7 Aug. 1991.

7. Fluoride medium-temperature electrolyser anode. U. N. Zusailov, RU 2118995, published on 20 Sep. 1998.

8. Anode for galvanic processes. A. V. Yuzhanina, L. A. Dronseiko and others, SU 1121327, published on 30 Oct. 1984.

9. Electrocatalytic method of synthesis of hydrocarbons and alcohols based on vegetable raw materials. V. N. Andreev, U. A. Antonova and others, RU 2471890, published on 1 Oct. 2013.

10. Methods for the electrolytic production of erythritol. Jonathan A., J. David, Daniel M., Peter M. U.S. Pat. No. 9,133,554, Publication Date 15 Sep. 2015.

11. Material for electrolyser electrode production. Kurtov V. D., Kosinov B. V., and others, RU 2282679, published on 27 Aug. 2006.

12. Pyrographite. Production, structure, properties. A. S. Fialkov, A. I. Bayer, and others. Success of chemistry, AS USSR, issue 1, 1965, Tome XXXIV, pg. 132.

13. https://doncarb.com/articles/katod-grafitovyy/, 26 Mar. 2017. 

1. A carbon-containing nonconsumable anode for electrolysis, which is different in that it is made of pyrocarbon (pyrographite).
 2. A non-consumable anode for electrolysis as per item 1, which is different in that the pyrocarbon is of UPA-3 grade. 